DE 101 55 470 A1 describes a method for the synthesis of propylene oxide by epoxidation of propene with recovery of unreacted propene, in which propene is recovered from at least a portion of an off-stream of the propylene oxide synthesis by (i) addition of nitrogen to the off-gas stream, (ii) compression and (iii) condensation of the resulting stream, (iv) subjecting the stream to gas permeation and (v) separation. During condensation, a gas stream comprising propene, nitrogen and oxygen is separated from a liquid stream and fed to gas permation. Addition of nitrogen is conducted so as to obtain a stream resulting from retentate of the gas permeation which has a low content of oxygen. Thus, formation of an ignitiable mixture is avoided.
EP 0 719 768 A1 describes a process for recovering an olefin and oxygen which are comprised in an off-gas stream obtained from catalytic reaction of the olefin with hydrogen peroxide. In this separation process, the off-gas stream is contacted with an absorption agent such as isopropanol. In order to avoid ignitiable gas mixture, an inert gas like methane has to be added.
EP 1 270 062 A1 describes a process for the recovery of combustible compounds of a gas stream comprising the combustible compounds and oxygen by selective absorption in a solvent. During absorption, the gas phase is dispersed in a continuous liquid phase of the solvent. As explicitly stated, an inert gas should be fed to the head zone of the absorption unit above the liquid level due to savety aspects. This addition of the inert gas is necessary to avoid the formation of an ignitiable mixture.
WO 2004/037802 A1 describes a method for continuously returning an olefin which has not been reacted with hydroperoxide in an olefin epoxidation reaction. The olefin is contained in an off-gas stream which is produced during the epoxidation. The method comprises (i) compressing and cooling the off-gas stream, (ii) separating the olefin from the off-gas stream obtained in (i) by distillation and (iii) epoxidizing the olefin separated in (ii) with hydroperoxide. In this method, it is not necessary to separately add an inert gas since for separating the oxiranes by distillation, an inert gas has been already added for controlling the distillation column.
U.S. Pat. No. 3,312,719 describes a process for oxidizing an unsaturated aliphatic hydrocarbon with a gas containing molecular oxygen, utilizing in this oxidation an excess of lower aliphatic hydrocarbon and recycling the unreacted lower aliphatic hydrocarbon after separation of the principal oxidation products therefrom. At least a portion of said lower aliphatic hydrocarbon is extracted from the main gas stream with a higher boiling hydrocarbon. Subsequently, the lower hydrocarbon dissolved in the washing liquid is blown out from the washing liquid using the gas containing molecular oxygen.
U.S. Pat. No. 6,712,942 B2 describes a process for working up a mixture comprising an alkene and oxygen, wherein oxygen is removed from this mixture by a non-distillative method. From the resulting mixture comprising the alkene, the alkene is separated by distillation. U.S. Pat. No. 6,712,942 B2 describes various possibilities of how to separate oxygen by a non-distillative method. According to one alternative, oxygen is burnt using a catalyst. According to another alternative, oxygen is burnt without a catalyst. As to possible catalysts for burning oxygen, Pd catalysts are disclosed which are supported on alumina. Also copper chromite catalysts are mentioned. According to yet another alternative of a non-distillative method, reaction of the oxygen with a suitable chemical compound is disclosed wherein oxydehydrogenation is explicitly mentioned. As catalyst useful for the oxydehydrogenation reaction, only a LiCl/TiO2 catalyst is specifically described, prepared according to an article by Xu and Lunsford (React. Kinet. Catal. Lett. 57 (1996) pages 3 to 11). It is explicitly stated in U.S. Pat. No. 6,712,942 B2 that, after a first separation of oxygen, the gas mixture should be brought in contact with a suitable solid such as finely divided copper on Mg silicate for further separation of oxygen.
U.S. Pat. No. 4,870,201 discloses a process for the production of nitriles from hydrocarbons by reaction with oxygen, air, or a gas enriched in oxygen relative to air, and ammonia in the presence of an ammoxidation catalyst. After catalytic dehydrogenation of the alkane to the alkene and subsequent ammoxidation, the obtained reaction mixture is quenched and the gas stream obtained is separated in a pressure swing adsorption unit having two adsorption beds. From the first bed, a gas stream is obtained comprising unreacted alkane, alkene and typically 1 to 2 percent by volume oxygen. Additionally, a stream is obtained from the first bed which comprises oxygen and optionally nitrogen and hydrogen. This stream is fed to a second adsorption bed from which a stream comprising oxygen and a stream enriched in hydrogen are obtained. At least a portion of the stream enriched in hydrogen and the stream comprising alkene and alkane from the first bed are subjected to a selective oxidation in order to remove the remaining oxygen. As catalyst suitable for the selective oxidation, noble metals and especially platinum or palladium on alumina are disclosed. Apart from that disclosure, U.S. Pat. No. 4,870,201 does not contain any further information regarding these catalysts. The stream which is obtained from the first adsorption bed and which is subjected to the selective oxidation typically comprises from 1.2 to 1.7 percent by volume propene, from 61.4 to 79.2 percent by volume propane and from 2.9 to 3.2 percent by volume oxygen.
U.S. Pat. No. 4,943,650 discloses a similar process. The stream which is subjected to the selective oxidation typically comprises about 1.5 percent by volume propene, from 88.8 to 90.7 percent by volume propane and less than 1 percent by volume oxygen, such as, e.g., 0.6 or 0.7 percent by volume oxygen.
U.S. Pat. No. 4,990,632 discloses a process for the production of oxides where a gaseous alkane is dehydrogenated to the corresponding alkene and the obtained alkene is reacted with a gas comprising air in a gas phase reaction to an alkylene oxide. Subsequently, the product stream is quenched in a liquid wherein a liquid phase comprising the alkylene oxide and a gas phase are obtained. The gas phase is fed to a pressure swing apparatus to remove, among others, oxygen. The gas stream thus obtained is subjected to a selective oxidation where the remaining oxygen is removed. Therefore, in the process of U.S. Pat. No. 4,990,632, there are two mandatory process stages in which oxygen is removed. The stream comprising propene, propane and oxygen, subjected to selective oxidation, typically comprises less than 2 percent by volume oxygen. As catalysts suitable for the selective oxidation, noble metals, especially platinum or palladium on alumina are disclosed. Apart from that disclosure, U.S. Pat. No. 4,990,632 does not contain any further information regarding these catalysts. The stream obtained from the pressure swing apparatus comprising propene, propane and oxygen typically contains about 60 percent by volume propene and about 30 percent by volume propane.
U.S. Pat. No. 5,929,258 discloses a method of manufacturing an epoxide wherein in a dehydrogenation step, a gas comprising an alkane is dehydrogenated and wherein the obtained gas comprises alkene and hydrogen. This gas is reacted with a further gas comprising oxygen in a gas phase reaction so that the alkene is epoxidized. As catalyst, a catalyst comprising gold is employed. Subsequently, the epoxide is separated wherein a gas comprising unreacted hydrogen and unreacted oxygen is obtained. Additionally, this gas can comprise by-products, unreacted alkane, unreacted alkene. Subsequently, oxygen and hydrogen are reacted with each other wherein a gas is obtained which contains unreacted alkane. Regarding the catalyst, U.S. Pat. No. 5,929,258 only contains the hint that this catalyst preferably contains a noble metal of group VIII such as platinum or palladium or, alternatively, ultrafine gold particles having a diameter of 10 nm or less. In the examples of U.S. Pat. No. 5,929,258, a platinum catalyst supported on alumina is disclosed.
WO 2004/033598 A1 describes a process for the removal of oxygen from a gas mixture comprising oxygen, at least one olefin, hydrogen, carbon monoxide and optionally at least one alkyne wherein the ratio of oxygen:hydrogen in the gas mixture is 1 part by volume of oxygen to at least 5 parts of volume of hydrogen, i.e., the volume ratio of oxygen to hydrogen must be smaller than or equal to 0.2, i.e. the hydrogen:oxygen ratio is greater than or equal to 5. Accordingly, examples 9 and 10 of WO 2004/033598 A1 disclose gas streams having a molar oxygen:hydrogen ratio of 0.0034, i.e. a molar hydrogen:oxygen ratio of 294, and examples 11 and 12 disclose gas streams having an oxygen:hydrogen ratio of 0.0052, i.e. a molar hydrogen:oxygen ratio of 192. The process comprises contacting the gas mixture with the catalyst in a reaction zone under conditions sufficient to oxidize at least a portion of the hydrogen and at least a portion of the carbon monoxide, without significant hydrogenation of the at least one olefin. The catalyst comprises at least one metal selected from the group consisting of the 10th group and the 11th group of the periodic table of the elements, the metal or oxide of the metal being supported on an oxide support, provided that where the catalyst comprises at least one metal or oxide of metal from the 10th group supported on an oxide support, the catalyst also comprises tin and provided that where the catalyst comprises at least one metal or oxide of metal of the 11th group, the oxide support is a zeolite. The gas mixtures subjected to the process of WO 2004/033598 A1 are typically obtained from steam cracking of hydrocarbons, dehydrogenation of paraffinic feedstock, conversion of methanol to olefins and auto-thermal cracking of hydrocarbons. The process of WO 2004/033598 A1 is particularly suitable for gas mixtures comprising from greater than 0 up to and including 60 percent by volume olefin. Advantageously, the process of WO 2004/033598 A1 enables oxygen to be removed from gas mixtures containing low levels of oxygen such as 2000 ppm or less of oxygen, and especially from gas mixtures having a low concentration of oxygen and a high concentration of hydrogen such as at least 10 percent by volume of hydrogen or for example greater than 40 percent by volume of hydrogen. Preferred catalysts according to the examples of WO 2004/033598 A1 contain platinum and tin supported on silica, the catalyst comprising at least 0.7 wt.-% of platinum and at least 1.87 wt.-% of tin.
Accordingly, the prior art describes, on the one hand, industrial processes such as dehydrogenation processes in which gas mixtures are obtained containing oxygen, hydrogen, olefin and optionally alkanes in mutual ratios which are fundamentally different from the gas mixtures obtained from epoxidation reactions such as epoxidation of propene. On the other hand, the prior art describes catalysts which do not meet the specific requirements of removing oxygen from gas mixtures obtained in epoxidation reactions such as epoxidation of propene.
Moreover, adsorption techniques described in the prior art have the major disadvantage that during adsorption, the explosive range of propene/oxygen mixtures is passed due to the increasing concentration of absorbed oxygen. Consequently, in order to avoid process risks, apparatuses used for adsorption techniques have to be constructed highly pressure resistant, thus causing high costs which in turn render the overall process economically undesirable.
Therefore, it is an object of the present invention to provide a process for producing propylene oxide in which an effective removal of oxygen from gas mixtures directly or indirectly obtained from the epoxidation reaction of propene is achieved.
It is a further object of the present invention to provide a process for producing propylene oxide in which heat integration is improved in specific reaction stages.
It is another object of the present invention to provide a specific catalyst for use in a work-up stage of a process for producing propylene oxide, in which work-up stage oxygen is effectively removed from a gas mixture.
It is still another object of the present invention to provide a work-up stage in a process for producing propylene oxide, in which work-up stage oxygen is effectively removed from a gas mixture comprising oxygen and propene wherein the disadvantages of absorption process are avoided.
It is still another object of the present invention to provide a work-up stage in a process for producing propylene oxide, in which work-up stage oxygen is effectively removed from a gas mixture comprising oxygen and propene by a specifically adapted catalyst in combination with a specifically adapted addition of hydrogen.
It is still another object of the present invention to provide a work-up stage as described above which can be also used for effectively removing oxygen from gas mixtures comprising an olefin and oxygen, the olefin being different from propene, wherein the disadvantages of absorption process are avoided.
It is still another object of the present invention to improve heat integration aspects of a propene epoxidation process.
It is still another object of the present invention to provide a work-up stage in a process for producing propylene oxide where methanol is used as solvent or part of a solvent mixture, wherein methanol is separated in the work-up stage having a degree of purity which allows for direct recycling into the process.
It is still another object of the present invention to provide a work-up stage in a process for producing propylene oxide where propene is used as starting material, wherein unreacted propene is separated in the work-up stage having a degree of purity which allows for direct recycling into the process.
It is yet another object of the present invention to provide a process for producing propylene oxide in which gas mixtures having too high an oxygen concentration are avoided.